1. Field of the Invention
The present invention relates to a positioning controlling apparatus, a positioning control method, a program for making a computer execute this method, an image forming apparatus, and a recording medium.
2. Description of the Related Art
In recent years, it is a very important task, from a viewpoint of image quality, to accurately move a paper conveying system by a desired distance and stop the system at this position, in an image forming apparatus such as a copying machine and a printer, particularly, in an image forming apparatus having an inkjet printing mechanism.
There has been developed a technique of decreasing the cost of generating a circuit and facilitating a change in control design, by digital control of software process using a low-cost general-purpose central processing unit (CPU) and a low-cost digital signal processor (DSP) in place of an analog control circuit.
A moved amount of a paper conveyor roller system that conveys recording paper in the above image forming apparatuses can be obtained at a relatively low cost, by obtaining information corresponding to a rotation angle of the conveyor roller, with a rotary encoder fitted to the conveyor roller. The encoder generates pulses with the edges settled beforehand. While the encoder includes an analog type and a digital type, the digital type encoder can stably obtain rotation angle information with a sensor. Precision of the obtained rotation angle information depends on resolution of the encoder.
FIG. 6 is an explanatory diagram of motor control of a paper conveying system performed by a conventional positioning controlling apparatus. In a paper conveyor belt apparatus 7, a drive roller 372 connected to a timing belt 470 from a motor 270 rotates to move a conveyor belt 371. A rotary encoder 571 is set on the drive roller 372.
A sensor 572 detects an output from a rotary encoder 571. A positioning controlling apparatus 17 outputs an obtained control instruction to a motor 270, based on position information detected by the sensor 572, and applies torque to the motor 270. The control instruction has a different format depending on a motor driver, such as a current instruction and a voltage instruction.
A control algorithm of this motor control includes a speed feedback control up to a position near a target value, and a position feedback control which is performed near a stop position. In the speed feedback control, the controlling apparatus measures a current speed and controls the current speed to make the difference between the current speed and a target speed zero.
In the position feedback control, the controlling apparatus multiplies a predetermined gain to a difference between the current position and the target position, thereby obtaining a target speed. The controlling apparatus performs a speed feedback based on the target speed, thereby simultaneously setting the speed and the position difference to zero.
However, near a stop position, behavior of the system based on the same instruction is different, depending on a twist of a rotation axis such as a motor axis, a drive roller axis, and a pulley to which each belt is applied. This behavior is greatly affected when the twist becomes large. Particularly, the behavior is different depending on a rotation direction of the system.
Torque is applied to the system in a direction returning from the twist. Therefore, it is not necessary to output an instruction to apply large torque in this direction. When torque larger than is necessary is applied, oscillation can be generated. On the other hand, relatively large torque needs to be applied in a direction opposite to the direction of returning from the twist. When this torque is small, time of converging to a target position can be delayed.
However, conventionally, it is not necessary to perform such a fine control in a sub-scan direction. This is because image degradation due to a “blur” of an image forming material such as ink at the image formation time, is more prominent than the positioning deviation, thereby overlooking the precision in positioning in the sub-scan direction. However, recently, ink for an inkjet printer has high granularity to meet a demand for an improved image quality. As a result, high precision is also required for paper feeding in the sub-scan direction.
In a conventional image forming apparatus such as an inkjet printer, a high-precision position deviation correction has not been particularly necessary as explained above. Therefore, there are few conventional examples of high-precision position correction. In other technical field, a positioning control technique in a servo control method for driving a feed rod of machine tool is proposed in Japanese Patent Specification No. 3271440. According to this servo control method for driving a feed rod of machine tool, in order to correct a positioning error due to a backlash, a state switching time is predicted by taking a delay in a position instruction and a position output into consideration, and a servo control is performed by switching an integration time constant.
However, the technique disclosed in Japanese Patent Specification No. 3271440 is applied for cutting metal or the like having high hardness in the machine tool and solves a similar problem in a technique assuming occurrence of a huge torque. Therefore, this technique is not suitable to control the behavior of a speed change near a stop position, as a method of correcting a positional deviation of a rotation axis of a general image forming apparatus in which a large torque like metal cutting does not occur.